The present invention relates to composite containers, and in particular relates to composite containers having releasable liners applied to the inner surface thereof.
Food and drink products and other perishable items are often packaged in tubular containers that are sealed at both ends. In the packaging of such perishable food products, a container is required that is rigid enough to retain its shape when subjected to reduced internal pressure created in the container while tightly sealing the food product to protect against deterioration, leakage, and contamination. Substantial economies, as well as environmental advantages, can be effected by the use of composite containers, as opposed to the traditional glass and metal containers. Composite containers typically comprise at least one body ply made from paper or other material that provides the main support for the container. The body ply may be formed by wrapping a continuous strip of the body ply material around a mandrel or passing the body ply material through a series of forming elements so as to wrap the material in a convolute shape around the mandrel. The tube is then cut into predetermined lengths at a downstream end of the mandrel and fitted with end closures to complete the container.
For hermetically sealing the container, a liquid impermeable liner ply is often adhered to the inner surface of the paperboard body ply. The liner ply seals the food product within the container and also prevents liquids, which may possibly contaminate the food product, from entering the container. Some liner plies are also gas impermeable, so as to not only prevent food product odors from escaping the can, but also prevent atmospheric air from entering the container and spoiling the food product. Thus, while the purpose of the body ply is to provide necessary structural properties to the container, the liner ply provides barrier properties necessary to protect and maintain the food product.
In addition, a label ply is typically included and adhered to the outer surface of the paperboard body ply. The label ply, which is typically a paper-based material, is a source of information that conveys product information, instructions, and regulatory compliance information. The label is also preferably decorative and aesthetically pleasing to the consumer, which enhances shelf appeal and increases consumer interest in the food product.
Certain food products create a vacuum within the container that occurs due to processing conditions. In particular, products that are deposited within the container that have a temperature greater than that of the room temperature will create a slight vacuum once the ends are placed on the container and the temperature of the products decreases to that of the room temperature. For example, relatively hot potato crisps can be deposited within the container and then sealed within the container by an end closure. As a result, hot air is trapped within the container. As the temperature of the air in the container decreases, the volume of the air decreases as well. Thus, a slight vacuum is formed within the container. Because of the structural design of the composite container, the application of vacuum to the interior of the container can introduce excessive stress to the paperboard body ply of the container, which often results in partial or complete inward collapsing of the container walls along the length or the container. This can result in an unacceptable appearance for the composite container or an unacceptable sealing of the product within the container. This product can be further exacerbated if containers packaged at a certain elevation are then shipped to a higher elevation (and thus lower ambient air pressure) which further increases the pressure differential between the inside and the outside of the containers.
U.S. Pat. No. 4,158,425, herein incorporated by reference, discusses problems associated with vacuum packaging food products in composite containers. To avoid the partial or complete collapsing of the paperboard body ply of the container upon the creation of the vacuum inside the container, the container according to the ""425 patent has an impermeable or hermetically sealed liner secured interiorly to the container body solely at the opposed ends thereof with the major length of the liner being free of the tubular body so as to allow an inward contracting of the liner without the introduction of excessive stresses to the container body itself. A vacuum or reduced pressure atmosphere within the liner causes an inward deformation of the liner into contact with the product substantially independently of the surrounding container body. Thus, the stresses which are transferred to the container body are at the opposed ends thereof, which are in turn rigidified by a pair of conventional end caps.
The ""425 patent, however, requires that the adhesive between the liner ply and the paperboard body ply be applied by a special adhesive applicator having a unique spiral design such that the adhesive is applied only at the respective ends of the composite container. By using the spiral design adhesive applicator, complexity is added to the production process as well as limiting the types of tubular composite containers that can be processed using that particular adhesive applicator. Although the ""425 patent provides an inwardly moving liner, it is desirable to provide an inwardly moving liner using conventional manufacturing techniques, such as standard roll adhesive applicators.
Moreover, composite containers that are designed for use with products subjected to a rapid vacuum application during processing may still place excessive stresses on the body ply because the vacuum application can create a vacuum between the liner and body ply as the liner ply is drawn inwardly. In this regard, many composite containers require a vent hole in the body ply for allowing the rapid passage of air between the unadhered portions of the liner ply and the body ply, which adds complexity to the manufacturing process.
Accordingly, there is a need in the industry for a container that hermetically seals food products, but that can be made using standard manufacturing techniques. At the same time, however, such a container would also be robust such that the container is capable of withstanding the rigors of packaging, shipping and storing products that create a vacuum within the container.
The composite container of the present invention balances the need for ease of manufacturing with the strength necessary to maintain a hermetic seal as the vacuum is created within the container by providing a flexible liner that is adhered to an adjacent tubular body wall using standard application techniques, but is capable of substantially releasing from the adjacent tubular body wall when the container is heated to a threshold temperature for a predetermined time interval. Thus, the released portion of the liner ply is free to move inwardly toward products contained therein. The flexible liner, which has at least one layer comprising a polymeric material, is adhered initially to the tubular body wall along the entire length of the container. When the container is heated, however, the adhesive begins to lose its adhesive ability. Also during the heating process, the polymeric material of the liner ply contracts, thus causing the liner to substantially withdraw from the tubular body wall. As such, the tubular composite container of the present invention requires no special adhesive application, which would otherwise add cost and complexity to the manufacturing process.
In particular, the composite container of the present invention comprises a tubular body wall having opposed ends and inner and outer surfaces. The flexible liner comprises an outer layer having inner and outer surfaces, wherein the outer surface of the outer layer is releasably adhered to the inner surface of the tubular body wall using a heat-releasable adhesive. The flexible liner also includes an inner layer adhered to the outer layer which is formed of a polymeric material that contracts upon being heated. In one advantageous embodiment, the inner layer comprises a polyethylene film. When the container is heated to the threshold temperature for the predetermined time interval, the liner substantially releases from the tubular body wall and is free to move inwardly towards any products that may be contained therein thus providing not only a hermetic seal but also support to the products. In this manner, if a vacuum is created by the products deposited within the container, the vacuum does not act directly on the tubular body wall, thus preventing inadvertent collapse of the body wall. Instead, the liner ply moves inwardly to substantially equalize the pressures on either side of the liner ply. When the container is opened, the liner can be moved away from the products such that the products may be removed. A label surrounds the outer surface of the body wall.
Associated methods also form a part of the invention and, according to one embodiment, include the steps of providing a tubular body wall formed of a paperboard material having opposed ends and a flexible liner adjacent an inner surface of the tubular body wall, closing one end of the tubular body wall, and heating the tubular body wall and the liner such that the liner substantially releases from the tubular body wall. In one embodiment, food products having a temperature greater than room temperature can be deposited within the flexible liner and the open end of the container can then be closed with an end closure. As the products deposited within the liner ply cool to the surrounding temperature, a negative pressure is created within the liner such that the liner is moved inwardly away from the inner surface of the tubular body wall and toward the food products. In another embodiment, the products can be deposited within the container before the heating step that causes the liner to substantially release from the tubular body wall.
Accordingly, and as is explained in more detail below, the present invention provides a new container capable of withstanding a negative pressure created therein which overcomes the disadvantages of conventional methods and containers, namely requiring special patterning of the adhesive between the liner ply and the tubular body ply. The present invention is particularly advantageous for food products that are deposited within the container at an elevated temperature, such as roasted peanuts or potato crisps. The new container is easy to manufacture, and can use conventional adhesive applicators and techniques. At the same time, however, the present container is capable of withstanding negative pressure created within the container so as to maintain a rigid shape, and provide a hermetically sealed container to prevent air and moisture from contaminating the products contained therein.